5, 6, 8, 11, 15, 20
The difference in 1st term and 2nd term = 6 - 5 = 1
The difference in 3rd and 2nd term = 8 - 6 = 2
The difference in 4th and 3rd term = 11 - 8 = 3
The difference in 5th and 4th term = 15 - 11 = 4
The difference in 6th and 5th term = 20 - 15 = 5
The differences are the counting numbers, 1, 2, 3, 4, 5 ... etc.
If we go on like this, we know the next number would be 6 added to previous.
The next to next would be 7 added to its previous. Thus, the numbers following this would be:
20 + 6 = 26
26 + 7 = 33
33 + 8 = 41
41 + 9 = 50
The extended list is:
5, 6, 8, 11, 15, 20, 26, 33, 41, 50 ... [goes on...]